This invention relates generally to control circuits for the selective actuation or firing of electrical loads such as ignitors used as electric fuseheads in blasting detonators and for the igniting of incendiary charges in pyrotechnic devices, etc. More specifically, this invention relates to electrical control circuits for energizing an electrically ignitable load while providing protection from inadvertent or accidental ignition.
Fusehead assemblies are used in many contexts such as blasting operations, seismic exploration, and for the actuation of passive restraint systems in automobiles. Each such fusehead assembly includes at least one electrical ignition device, such as a fusehead, disposed in ignition relationship with one or more explosive charges. In all of these applications, it is important for the electrically ignitable load to be promptly actuated when desired, while at the same time for the load to be protected from inadvertent or accidental ignition.
In blasting operations and in seismic exploration, explosive charges are usually detonated from a remote firing point to ensure operator safety. An electrical firing signal is transmitted to a detonator which instantaneously or after some predetermined time delay explodes and ignites a main explosive charge.
Usually, an electric fusehead is ignited by an electrical current passing through a fuse wire (bridge wire) or metallic film constituting a resistive load. When sufficient electrical current passes through the fuse wire or metallic film, joule heating takes place and the temperature of the wire or film rises sufficiently to ignite a chemical composition disposed in contact or in close proximity with the wire or film. The heat generated from the ignition of the chemical composition is then utilized to ignite a sequence of pyrotechnic and/or explosive charges which in turn ignite or detonate the main explosive charge. The electrical energy for igniting the fusehead is usually obtained from a battery, pulse generator, AC power supply or the discharge of a capacitor.
To ensure operator safety during the storage and installation of explosive charges utilizing electrical fusehead detonators, it is essential that ignition of the fusehead does not occur until an authentic firing signal is generated. However, the environment within which electric fuseheads are stored, transported, installed, and operated usually includes various sources of electrical energy that are capable of inducing an accidental or inadvertent ignition of the fusehead. For example, typically during blasting operations involving large numbers of personnel, batteries, and electric fuseheads, there may be accidental or unauthorized direct connection of the lead wires of a fusehead to a battery or other power source. In addition, power wiring located in the vicinity of a blasting site may electromagnetically induce sufficient current to ignite an electric fusehead. Furthermore, currents may be induced in the lead wires of a fusehead from electromagnetic radiation from communication transmitters, radar installations, and the like. Another potential source of induced firing current is static electric discharge from the loading of a dry granular explosive. For automobile passive restraint systems, the electric battery in the automobile constitutes a source of electrical energy for accidental connection during maintenance or testing of the automobile.
The degree of safety associated with a given electric fusehead installation depends upon both the sensitivity of the fusehead to ignition by spurious sources of electrical energy and upon the probability that such spurious sources will be encountered. Various approaches to the problem of enhancing the degree of safety associated with the operation of electric fuseheads have been taken. One such approach has been to decrease the sensitivity of an electric fusehead by designing the fusehead so as to require very high firing currents for igniting the pyrotechnic chemical disposed adjacent to the fuse wire or film which is heated by the firing signal. This approach requires the use of heavy and expensive wiring and requires the use of power sources providing high energy levels. In addition to the increased expense associated with this approach, this approach fails to provide adequate safety for some operations, such as in mining where dry granular explosives are loaded by compressed air.
One approach to the safe handling of fusehead igniters is set forth in a prior copending application of Jones, et al for an Electric Igniter filed May 15, 1979 and bearing Ser. No. 39,443. That application teaches a system for actuating a plurality of electrically actuable igniters by utilizing a continuous length of insulated wire looped around a transformer core having a movable portion. The movable portion is utilized to assemble a firing configuration using a multiple igniter looped therethrough.
Another approach is shown in a co-pending application of Andrew Stratton bearing Ser. No. 4,265 filed Jan. 17, 1979. This invention relates to linking an ignitable load such as a fusehead to a source of power by coupling through a transformer constructed to provide a substantial leakage inductance associated with the secondary winding. In this manner input electrical energy having only a predetermined magnitude in frequency characteristic will actuate the load.
A further approach in the safe handling and actuation of electric fuseheads has been to incorporate tuned circuits for selectively energizing an electric fusehead in response to an input electrical signal having a predetermined frequency. For example, U.S. Pat. No. 3,762,331 teaches the use of a voltage step-down transformer in combination with capacitors and an inductor for selectively operating an electric fusehead at a frequency of approximately 10 KHz. The voltage ratio of the step-down transformer is large (on the order of 100:1) so as to increase the voltage level required for firing thereby decreasing the sensitivity of the fusehead to spurious input voltages even if the input voltage is within the correct frequency range. A series input capacitor is utilized to block accidental ignition from spurious DC voltages and to attenuate low frequency AC signals (50-60 Hz. power frequencies). A shunt capacitor is coupled across the primary of the transformer to bypass higher frequency radio signals which may appear across that winding. A series input inductor is utilized to match input line impedances and to attenuate higher frequencies. Coupling transformers for use in such protective systems have been designed so that magnetic saturation of the transformer core provides increased protection against improper fusehead ignition at AC power frequencies (50-60 Hz.).
The use of a transformer coupled electric fusehead is illustrated in British Pat. No. 1,235,844, published in 1971. This British patent shows a pot-shaped core transformer coupled AC input for an electric fusehead which ignites in response to a firing signal having a frequency of 330 Hz. Protection from higher frequencies is achieved through transformer core loss attenuation.
Although the use of transformers having large step-down ratios are reasonably effective in protecting electric fuseheads, their usefulness is limited because they are impractical. Typically, fusehead firing voltages on the order of 100 volts are required. Such voltages are not always available or not commercially realistic. Furthermore, for use in complex blasting operations the use of large individual detonator firing signal voltages may require excessive large overall firing voltage for a series connection of a plurality of the circuits. Furthermore, transformers having large step-down ratios are often bulky and therefore difficult to handle. In addition, such transformers provide little protection against high energy static discharges typically encountered in blasting operations. Thus, these transformer circuits remain vulnerable to accidental ignition during transport, storage and connection into a blasting arrangement including multiple devices. Thus, there is still a need for a more simplified and commercially feasible control circuit for electric fuseheads providing protection from accidental or inadvertent ignition during manufacture, transport, storage and connection into a blasting arrangement.